1. Field of the Invention
The present invention relates to an epitaxial wafer and a method of producing the same. More specifically, the present invention relates to an epitaxial wafer employing no silicon wafer having a complete SOI structure, allowing high-precision thinning of the epitaxial wafer, and achieving cost reduction. The present invention also relates to a method of producing the epitaxial wafer.
2. Description of the Related Art
A SOI (silicon on insulator) wafer is known, in which an implanted oxide film is formed in a surface layer of a silicon wafer, and thereby an active layer having monocrystalline silicon is formed on a wafer surface side of the implanted oxide film. Further, a SIMOX (Separation by IMplanted OXygen) wafer has been developed as one type of the SOI wafer. The SIMOX wafer is provided with an ion-implanted layer, which is formed by ion-implanting oxygen in the surface layer of the silicon wafer from the wafer surface. After the forming of the ion-implanted layer, the silicon wafer is heat-treated, and thereby the ion-implanted layer is provided as an implanted oxide film (implanted silicon oxide film).
An epitaxial SIMOX wafer, which is a SIMOX wafer having an epitaxial film grown on a surface thereof, is heavily used as a wafer for CIS (CMOS Image Sensor), which is one type of an image sensor (Patent Literature 1, for example). The image sensor is a device that captures image information by utilizing photosensitive characteristics of a semiconductor. CIS absorbs light that has captured an external image, and integrates photocharge using a photodiode, which is a light receiving element. A device is formed on the surface of the epitaxial SIMOX wafer for CIS in a device forming process. A silicon wafer is then attached to the surface of the epitaxial film. Subsequently, the SIMOX wafer is ground and polished, or etched, from a rear side thereof, and thus is reduced in the thickness. Thereby, a wafer is provided, in which a device is implanted on the rear side of the epitaxial film (a location sandwiched by the attached wafer).
Oxygen is ion-implanted herein under conditions of a temperature of 200° C. to 600° C.; an implantation energy of 20 keV to 220 keV; and an ion implantation amount of 1.5×1017 atoms/cm2 to 2×1018 atoms/cm2. The implanted oxide film is used as a polishing stopper or an etching stopper in a process in which the wafer is thinned from the silicon to the implanted oxide film. Material properties are capitalized on herein, including a difference in hardness between oxide silicon and silicon, which causes a change in polishing resistance of the wafer; and a difference in an etching rate between oxide silicon and silicon, relative to an etching solution, which causes a change in an etching speed.    [Patent Literature 1] Japanese Patent Laid-open Publication No. 2005-333052
As described above, the conventional CIS wafer has the SIMOX wafer as a main body and the oxygen ion implantation amount of 1.5×1017 atoms/cm2 to 2×1018 atoms/cm2. Thus, when ions are implanted presumptively when particles are deposited on the wafer surface, a defect occurs in which the implanted oxide film breaks in a portion associated with the particles. The defect causes a pit in the wafer surface when the ion-implanted layer is annealed at high temperature. Specifically, when the ion-implanted layer changes to the implanted oxide film in the high temperature annealing, a film thickness increases in a portion excluding the defective potion, along with oxidation of silicon. Accordingly, a pit is formed in the portion of the silicon wafer, the portion being associated with the defective portion. Consequently, a pit is formed in the surface of the epitaxial film at the time of epitaxial growth, thus causing an increase in surface detects of the epitaxial film. The problem occurs since the silicon grows in accordance with a shape of the wafer surface.
Further, when the epitaxial SIMOX wafer is produced, oxygen ions are not implanted in a chamfered portion (external peripheral portion) of the wafer. Thus, no implanted oxide film exists in the external peripheral portion of the wafer. Consequently, unevenness in temperature in the external peripheral portion of the wafer increases the thickness of the external peripheral portion of the wafer, which has no implanted oxide film, of the epitaxial film, thus forming a slip in the external peripheral portion of the wafer.
In addition, the conventional CIS wafer has the epitaxial film grown on the surface of the SIMOX wafer having the implanted oxide film. For the CIS wafer, it is thus required, according to a SIMOX wafer production method, that the ion-implanted layer be heat-treated at a high temperature of 1,300° C. or higher and that high temperature annealing be performed to form the implanted oxide film formed from a silicon oxide film. Consequently, expenses are increased for a high temperature annealing device, particularly for a SiC member, thus leading to a sharp increase in production cost of the epitaxial SIMOX wafer.